The dielectric loss of molecular glasses was investigated from the glass transition temperature $T_g$ down to 2 K, for glycerol down to 0.1 K, in the frequency range 50 Hz–20 kHz. The systems studied here do not exhibit a $\beta$ process. Three distinct relaxation regimes are identified, namely nearly constant loss (NCL), thermally activated dynamics in asymmetric double well potentials (ADWP), and tunneling. The NCL $(0.35<T∕T_g<1)$, described by ${\epsilon }^{″}\left(\nu \right)\propto {\nu }^{-\gamma }$ with $\gamma =0.09-0.21$, exhibits the characteristic exponential temperature dependence. The ADWP dynamics $(0.05<T∕T_g<0.35)$ resembles that of silica. At lowest temperatures (2–6 K) the tunneling plateau is reached. This is associated with a power law spectrum revealing a universal positive exponent that agrees well with that reported for ionic, inorganic and polymeric glasses. The so-called “plateau strength” $C=(2∕\pi )\bullet \tan \delta =P{\mu }^2∕\left(3{\epsilon }_0{\epsilon }_r\right)$ is determined.

• Received 22 June 2005

DOI:https://doi.org/10.1103/PhysRevB.72.174203